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Hawaiian Islands Synoptic Discussion and Guidance

197
FXHW60 PHFO 190126 CCA
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
325 PM HST Thu Jul 18 2019

.SYNOPSIS...
High pressure far to the north will provide trade winds into next
week, with wind speeds easing somewhat over the weekend. Clouds and
showers riding in with the trades will favor windward and mountain
areas, with showers most frequent during nights and mornings. An
area of especially showery clouds will arrive overnight, bringing
showers to many windward communities before departing Friday. A few
light showers may briefly spread to otherwise dry leeward areas,
with the exception being leeward Big Island, where clouds and a few
showers are expected each afternoon and evening.

&&

.DISCUSSION...
High pressure will remain entrenched far N and NE of the islands
over the next week, supporting a seasonable trade wind weather
pattern. The high will support locally strong ENE trade winds
initially, with wind speeds easing slightly by Saturday as it
weakens a little. With trade winds blowing, clouds and showers will
focus over windward areas, but a few brief showers will spread
leeward on the smaller islands. On the Big Island, this is the wet
season for the leeward slopes, where afternoon and evening clouds
can be expected (nearly daily) to drop a few showers. Having said
that, today has been a bit of an exception, with only patchy
cloudiness noted.

The leading edge of an area of open-celled showery cumulus clouds is
moving in to windward Big Island and Maui from the E, and will bring
a statewide increase in windward cloud and shower coverage
overnight, especially as compared to the current mostly dry
conditions. A few showers will spread leeward with the gusty trade
winds, but accumulations in general will be limited as models depict
this moisture moving W of the islands on Friday, and because the low-
levels will be capped at about 7000 feet. Latest GOES-17 satellite
imagery reinforces this notion, as the incoming showery low cloud
field transitions to a more stable cloud field in an area about 300
to 800 miles E and NE of the islands.

About 800-1000 miles to the E and NE, the cloud field becomes more
showery, with GFS indicating these clouds leading to an increase in
low-level moisture that arrives on Saturday. Hard to bank on the
details of that verifying, with the general expectation being that
the trade winds will deliver periods of clouds and showers to
windward areas, especially during nights and mornings.

Generally speaking, the pattern aloft will feature low pressure to
the NW and a ridge to the SE, with the low acting to occasionally
spread high clouds of varying thicknesses over the islands.
Currently, water vapor imagery shows a weak low aloft centered about
400 miles N of the islands, with an associated E-W oriented trough.
Afternoon soundings indicate that this feature is promoting
instability above a strong low-level subsidence inversion based near
7000 feet, with the unstable layer starting at 12000 feet on Kauai.
With the unstable layer based near 15000 feet on the Big Island,
surface-based air parcels are not expected to be able to tap into
the instability, and upslope convection is not expected, even as the
trough aloft becomes N-S oriented over the area over the weekend.
With the low-level inversion remaining in place, rainfall amounts
will be limited, even as showery patches move through with the trade
winds.

&&

.AVIATION...
A high will remain nearly stationary far northeast of the main
Hawaiian islands and maintain locally strong east winds over the
islands through Friday. AIRMET TANGO is in effect for moderate
turbulence below 7,000ft and this AIRMET will likely continue into
Friday.

The low level winds will carry some clouds and showers over east
sections of the islands. An area of broken low clouds moving over
the islands from the east has produced an increase in cloud cover
over east sections of the Big Island of Hawaii and east Maui but
little in the way of low ceilings or visibilities. These clouds may
become more active this evening and produce localized MVFR
conditions over east sections of the islands. AIRMET SIERRA for MTN
OBSC could be required. The area of showery low clouds should clear
the islands Friday afternoon.

&&

.MARINE...
High pressure north of the state will maintain fresh to strong trade
winds through Friday. The high is expected to weaken some over the
weekend, which will help the winds ease slightly. Trade winds may
drop to moderate to fresh speeds early next week for a brief day or
two.

The Small Craft Advisory (SCA) continues through Friday afternoon
for all waters in response to the trade winds. The forecast calls
for the winds to start to weaken Friday night, which will drop the
SCA back to the typical windier areas around Maui and the Big Island
for the weekend.

Rough and choppy surf is expected to hold steady along east facing
shores at heights just below the advisory threshold through Friday.
As the winds ease over the weekend, surf is expected to drop
slightly. Some small background south and southwest swells are
expected through the weekend. An increase in surf along south facing
shores is possible early next week as a couple small southwest
swells arrive in the islands.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters-Maui County Leeward Waters-Big Island Windward
Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Donaldson
MARINE...Kino

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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